Polypropylene fiber is hydrophobic but can be converted into a hydrophilic one by modifying its surface with polar groups. It is known in the art to achieve this result by chemically modifying the surface. However, topical chemical applications are not entirely satisfactory as they are not durable, and other types of surface modifications may need extra processing steps and tend to be expensive. In addition, some of these modifications age with time, especially in the presence of 31% potassium hydroxide (KOH) electrolyte solution used in alkaline batteries. An alternative and improvement over chemical modification is to directly melt blend an additive with polar groups that would migrate to the surface to render the polypropylene hydrophilic.
The invention provides such a process for producing a composition comprising polypropylene mixed with a hydrophilic polar compound including functional sites selected from the group consisting of carboxyl, hydroxy, ether or ester moieties. The process involves melting polypropylene granules with the hydrophilic compound, preferably maleic anhydride modified polypropylene (MA-PP). Fatty acid dimers and monomers, as well as polyvinyl alcohol and polyethylene glycols can be incorporated into the compositions. Surfactants and amorphous compounds such as EPDM, amorphous polypropylene, polybutylene blends and other low crystallinity polymers may also be added.
In general, use of maleic anhydride to modify polyolefins is known in the art, in particular, it is used to form copolymers with other olefins to produce polymers that absorb water. In most applications the maleic anhydride-polypropylene is used as a compatibilizer in blends of polypropylene with nylon, and polypolypropylene with fillers to improve the properties of the product. In both these situations the maleic anhydride-polypropylene remains inside the bulk of the blends and not at the surface.
U.S. Pat. Nos. 5,677,377 and 5,677,378 to Hasegawa et al., U.S. Pat. No. 5,230,963 to Knoerzer, U.S. Pat. No. 4,764,546 to Mitsuno et al., U.S. Pat. No. 4,708,918 to Fitzgerald et al. and U.S. Pat. No. 3,966,72 to Gaylord are representative of disclosures which include maleic anhydride modified polypropylene.
In particular, U.S. Pat. No. 4,957,968 to Adur et al. discloses use of maleic anhydride-polypropylene, ethylene propylene diene rubber and polypropylene in adhesive applications to the surfaces such as paper and aluminum. The melt flow rate of the Adur compositions are low, are partially cured and are not used for fiber and nonwoven applications.
The criteria used to achieve durable wettability are based on three factors: (1) materials that have hydrophilic groups and have a limited solubility in water, unlike the water-soluble surfactants that result in temporary wettability; (2) materials that have a viscosity lower than the matrix resin so that they can move to the surface as exhibited in the blends based on the lower melt flow rates; and (3) an amorphous co-additive which is blended to reduce the crystallinity and crystallization rate of the polypropylene to allow the wettability additives to migrate to the surface layers.
The wettable polypropylene compositions of the invention meet all these criteria by providing a mixture of unmodified polypropylene with a hydrophilic polar compound which includes functional sites selected from the group consisting of carboxyl, hydroxy, ether or ester moieties. The polypropylene composition produced is wettable, having a contact angle of less than or equal to 75.degree. which is an improvement over unmodified polypropylene films exhibit a contact angle of 100 to 105.degree..
There are several applications for the wettable or hydrophilic polypropylene of the invention, in particular in combination with a paper-based product. These include printable polypropylene for packaging, hygiene products and automotive and industrial application. Other applications include the battery market. The rechargeable battery separator market is in the process of transitioning from nylon 66/nylon 6, the material from which the separators are presently made from, to polypropylene because the nylon-based product undergoes degradation by the electrolyte. Such degradation decreases the overall cell lifetime. Polypropylene resists such degradation, however it needs to be permanently hydrophilic to be suitable for this application. Hence there is a need to develop a permanently wettable polypropylene by directly incorporating additives or polymeric components during melt processing. The modified polypropylene can be used to make wettable spunbonded and/or melt blown nonwovens.
An object of the invention is to develop wettability of polypropylene by using an additive with polar groups that wet with water and KOH solution without dissolving in it to prevent leach out.
A further object of the invention is to provide a wettable polypropylene that exhibits a contact angle of at least 70 to 75.degree. (similar to nylon), preferably lower, and resists the effect of 31% KOH used as an electrolyte in rechargeable batteries.
Another further object of the invention is to provide a wettable polypropylene which is formed into a nonwoven having a wicking of 3 cm or greater.
A specific object of the invention is to provide a blend of polypropylene containing the additives that can be converted into a fibrous material by melt blowing it or spinning fibers.
A more specific object of the invention is to provide a wettable polypropylene that can be used in battery separators, absorbent and hygiene products.
Another object of the invention is to achieve a wettable polypropylene extrusion coating on paper or paperboard for use in packaging applications where stiffness and printability is important.
Another object of the invention is to achieve a polypropylene which acts as a polar tie layer in laminates and there by eliminating the expensive tie layer of specialty resins.